The Blessed Virgin Mary
The Immaculate Conception
From the first instant of her conception, Mary was preserved free from original sin — by a special grace, in view of the merits of Jesus Christ. She is redeemed too; she was rescued by being kept from the pit rather than pulled out of it.
“Full of grace” — the angel's greeting
Gabriel addresses Mary with a title, not just a description: kecharitōmenē, a perfect passive participle meaning ‘you who have been (and remain) fully graced.’ A soul completely filled with grace has no room left for sin.
The new Eve and the enmity of Genesis 3:15
God places complete enmity between the woman and the serpent. Sin is the serpent's domain; a total enmity implies she was never under his power. The Fathers saw Mary as the ‘new Eve’ whose obedience undoes the first Eve's disobedience.
A fitting dwelling for God
Mary is the living Ark who carried God's presence in her womb. Just as the Old Testament Ark had to be built of the purest materials and could not be touched by the unclean, it is fitting that the Mother of the All-Holy be kept immaculate.
Common objections
“Romans 3:23 says ALL have sinned.”
That is Paul's summary of the human race's need for Christ; Scripture already names exceptions (infants, Jesus himself). Mary is the supreme exception — not by her own power but by Christ's grace applied to her at conception.
“Mary called God her Savior (Luke 1:47), so she was a sinner.”
Exactly — she IS saved by God. Preventing someone from falling into the pit is a greater act of saving than pulling them out afterward. Being preserved from sin by Christ's merits is still being saved by Christ.
Scripture quoted verbatim from the World English Bible (public domain).